Most gas laser devices such as helium neon type lasers utilize an aluminum cold cathode. This cathode is usually of cylindrical shape and longitudinally overlaps within a glass envelope the capillary discharge tube. In some of the more recent designs, the aluminum cold cathode has been mounted off a metal end plate closing the cathode end of the surrounding glass envelope. The mounting can be by springs or flexible leads between such end plate and the cathode.
The foregoing type designs place certain restrictions on the dimensioning of the components within the glass envelope. The end plate for the glass envelope at the cathode end of the envelope is generally of a nickel-iron alloy for effecting proper sealing with the glass of the envelope. Further, the coefficients of thermal expansion of this alloy and glass can be made essentially the same so that proper sealing will be maintained over wide temperature variations. On the other hand, the presence of a nickel-iron alloy end plate at the cathode end of the envelope can result in electron emission from the end plate itself because of the proximity of the cathode electrode. Such emission is very undesirable since the nickel-iron alloy has the property of sputtering at very low current densities and thus the useful life of the laser tube is drastically reduced.
The foregoing problem can be solved by physically positioning the aluminum cathode itself sufficiently far from the end plate to insure that little or no electron emission occurs from the end plate surface but that all emission takes place from the inside surface of the aluminum cathode. Such positioning of the cathode, however, prevents full utilization of the space in the envelope body between the end mirrors defining the resonant cavity. As a consequence, maximum active gain length is not utilized.